During the past decade, there has been some concern that the alternating electric and magnetic fields in the vicinity of electrical power transmission lines may be carcinogenic or otherwise harmful to humans and animals. For example, there appears to be a greater than expected incidence of leukemia in children living near pole transformers. The cause is believed to be due to a disabling, by the 60 Hz magnetic field, of the body's immune system rather than to initiation of the disease. It is suspected that minute electrical currents, induced by time-varying magnetic fields within the body, could confuse the immune system's ability to recognize cancer cells. The damage done to the immune system is temporary; presumably the immune system becomes effective immediately upon removal of the field. In contrast, the continuous fields generated by dc powered appliances are superimposed upon the Earth's magnetic field by vector addition or subtraction; there is no evidence suggesting harmful biological effects of dc fields of magnitudes comparable to that of the Earth's magnetic field.
The relatively intense ac fields produced by hair dryers and toasters have less effect upon the progress of a disease, because in their typically occasional use the immune system is apparently disabled for only brief periods. Electric blankets are another matter.
Electric blankets and heating pads are particularly pernicious because the body can be so close to potentially harmful periodically varying fields for a substantial portion of one's daily life. A paper by Wertheimer and Leeper entitled "Possible Effects of Electric Blankets and Heated Water Beds on Fetal Development" appearing in Bioelectromagnetics, Vol. 7, pp. 13-22 (1986) shows a correlation between the incidence of birth defects and the use of electric blankets.
The patent literature describes numerous means for heat control of electric blankets, but those which I have examined use an open-loop thermostat which senses and acts upon the ambient temperature of the room. The blankets may have embedded thermostats distributed in series with the heater wire, but these do not function unless there is an anomalous "hot spot".
The heating pads which I have examined have embedded thermostats which exert active control. A 4-position switch in the line cord allows the operator the selection of LOW, MEDIUM or HIGH temperature settings. During the "ON" period, heating pads typically draw 0.4 ampere regardless of the setting of the selector switch.
The current drawn by an electric blankets sized for a twin bed is approximately 1.1 ampere (corresponding to 140 watts) during the "ON" period. Thus, due to the difference in magnitude of the currents alone, the resulting magnetic field from an electric blanket is more than double that of a heating pad.
The thermostat and ON-OFF switches used in heating pads and blankets are generally single-pole. This is unfortunate because the entire heating element can float at high line voltage if the line plug is improperly polarized. In this situation, the electric field emanating from the appliance is worse when it is OFF than when it is ON.
The resistive heating element in contemporary blankets and pads is either a helical wire wound over a fiber core or a positive temperature coefficient (PTC) plastic strip bonded along its length to low resistance conductors connected to the power line. Both types of heaters are sheathed with an insulating plastic cover. The resulting cable is distributed in a serpentine configuration. It is the contemporary practice to make the electrical connections to opposite ends of the heater wire for both helical wire and PTC type heating cables. This causes the currents in the conductor wires to flow in the same direction and the stray magnetic field is reinforced.